We are proposing to use tracer kinetic methodologies to ask questions about the factors and processes that are important for accumulating and maintaining all-trans retinoic acid within tissues. The general question we propose to ask is: How do tissues in the intact animal acquire and maintain the all-trans retinoic acid need for regulating building on and further developing experimental and analytical methodologies and kinetic modeling approaches for the study of retinoic acid metabolism which we successfully used in the rat model. All of the studies which we are proposing in the project are focused on exploring and/or confirming one hypothesis regarding retinoid transport and metabolism. This hypothesis is that cells and tissues regulate intracellular retinoic acid concentrations and that all retinoid homeostatic mechanisms which the body are focused on maintaining intracellular retinoic acid concentrations. We believe that this is the cardinal imperative for regulating all retinoid transport and metabolism within the body. Our studies consist of four Specific Aims. First, we propose to validate the tracer kinetic methodologies which we developed and used in the rat model for the mouse. Then, using different strains of induced mutant mice which either have markedly different patterns and/or levels of circulating retinol, retinyl esters and retinoic acid or which lack a protein proposed to be importantly involved in retinoic acid synthesis within cells, we propose to employ these tracer kinetic approaches to ask how these differences influence retinoic acid turnover and metabolism in plasma and tissues of the mutant mice. These studies will shed light on our hypothesis that within the body the homeostatic mechanisms for regulating retinoid transport and metabolism are geared towards maintaining constant intracellular retinoic acid concentrations.